"I knew nothing about boxing when I started really," Gyllenhaal in the clip. "I knew that in order for me to pull it off in any way, I was just going to have to learn how to do it. When I set my mind to something I just won't stop until I do it. It's something I pride myself on. It's something I hate about myself. By the end I felt like I could do it."

Gyllenhaal fully committed, gaining 15 pounds of muscle for the role.

The movie follows fictional Junior Middleweight champ Billy Hope (Gyllenhaal) as he fights his way to a championship belt only to lose everything.

Rachel McAdams stars as Billy's caring wife Maureen, who is gunned down in front of him by another fighter wanting a shot at Billy's belt. Maureen's death sends Billy into a self-destructive, drug-addled spiral that leads the courts to take away his daughter (Oona Laurence). Making matters worse, Billy's income gets cut off after a doping scandal forces officials to suspend his boxing license for a year.

Left with nothing, Billy turns to former boxer Titus "Tick" Wills (Forest Whitaker) to rebuild him into the champ he once was and defeat the man responsible for his wife's death.

Southpaw opens July 31.

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Jake Gyllenhaal through the years

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Watch Jake Gyllenhaal get super buff for 'Southpaw'

Credit: Weinstein Company via ETOnline

NEW YORK, NY - NOVEMBER 18: Jake Gyllenhaal attends the meet & greet for the Manhattan Theatre Club production of 'Constellations' at the MCC Rehearsal Studios on November 18, 2014 in New York City. (Photo by Walter McBride/WireImage)

NEW YORK, NY - NOVEMBER 25: Actor Jake Gyllenhaal is seen on November 25, 2014 in New York City. (Photo by SBN/Star Max/GC Images)

NEW YORK, NY - NOVEMBER 03: Jake Gyllenhaal attends AOL's BUILD Speaker Series: Jake Gyllenhaal Discusses His New Film 'Nightcrawler' at AOL Studios In New York on November 3, 2014 in New York City. (Photo by Steve Mack/FilmMagic)

NEW YORK, NY - OCTOBER 29: Jake Gyllenhaal is seen filming 'Demolition' in Hudson River Park on October 29, 2014 in New York City. (Photo by Alessio Botticelli/GC Images)

NEW YORK, NY - OCTOBER 29: Jake Gyllenhaal and director Jean-Marc Vallee are seen filming 'Demolition' in Hudson River Park on October 29, 2014 in New York City. (Photo by Alessio Botticelli/GC Images)

NEW YORK, NY - OCTOBER 27: Actor Jake Gyllenhaal attends the 'Nightcrawler' New York Premiere at AMC Lincoln Square Theater on October 27, 2014 in New York City. (Photo by Noam Galai/WireImage)

NEW YORK, NY - MAY 05: [EDITOR'S NOTE: Image has been digitally processed] Jake Gyllenhaal and Maggie Gyllenhaal attend the 'Charles James: Beyond Fashion' Costume Institute Gala at the Metropolitan Museum of Art on May 5, 2014 in New York City. (Photo by Andrew H. Walker/Getty Images)

NEW YORK, NY - SEPTEMBER 11: Jake Gyllenhaal attends Annual Charity Day Hosted by Cantor Fitzgerald and BGC at Cantor Fitzgerald on September 11, 2014 in New York City. (Photo by Michael Loccisano/Getty Images for Cantor Fitzgerald)

TORONTO, ON - SEPTEMBER 06: Actor Jake Gyllenhaal (L) and director Jean-Marc Vallee attend the HFPA & InStyle's 2014 TIFF celebration at the 2014 Toronto International Film Festival at Windsor Arms Hotel on September 6, 2014 in Toronto, Canada. (Photo by Tommaso Boddi/WireImage)

TORONTO, ON - SEPTEMBER 05: Jake Gyllenhaal at the 'Nightcrawler' Press Conference at The Fairmont Royal York Hotel on September 5, 2014 in Toronto, Ontario. (Photo by Vera Anderson/WireImage)